Arthurian knight possessed after festival (7)
I believe the answer is:
galahad
'arthurian knight' is the definition.
(I've seen this before)
'possessed after festival' is the wordplay.
'possessed' becomes 'had' (to have something is to possess it).
'after' says to put letters next to each other.
'festival' becomes 'gala' (a gala is a festival or party).
'had' after 'gala' is 'GALAHAD'.
(Other definitions for galahad that I've seen before include "Chivalrous knight" , "Son of 9" , "principled chap" , "extremely honourable man" , "Noble knight" .)